Manufacture of carbon filaments

ABSTRACT

The production of carbon filaments by the pyrolysis of filamentary material, such as filaments of cellulose and polymers or copolymers of acrylonitrile, wherein the heating of the filaments above 900* is conducted in the presence of a carbon gettering material in a protective atmosphere. The carbon gettering material is preferably provided by the carbonization of cellulosic paper in which the filaments are wrapped during the heating above 900* C.

United States Patent inventors Kenneth G. 0. Dixon Ashtead; Richard Malcolm Gill, Pinner; Donald R. Lovell, London, all of England Appl. No. 795,718 Filed Jan. 31, 1969 Patented Nov. 9, 1971 Assignee Morgenite Research & Development Limited London, England Priority Feb. 2, 1968 Great Britain 5353/68 MANUFACTURE OF CARBON FILAMENTS 4 Claims, No Drawings [Sol References Cited UNITED STATES PATENTS 3,285,696 I 1/1966 Tsunoda 23/2091 3.297.405 l/l967 Sperk et al... 23/209] 3,305,3l5 2/1967 Bacon et al. 23/209.l 3,412,062 ll/l968 Johnson et al. 23/209. l X 3.508.871 4/l970 Cory 23/2691 Primary Examiner-Edward .l. Meros Attorney-Plumley. Tyner & Sandt MANUFACTURE OF CARBON FILAMENTS BACKGROUND OF INVENTION This invention relates to the production of carbon filaments having a very high modulus of elasticity.

Carbon filaments having a very high modulus of elasticity, for example above l0 p.s.i. and an ultimate tensile strength above 150,000 p.s.i. are known and find important use as structural materials when incorporated in matrices such as plastics and metal. I

The starting material for the production of such carbon filaments is essentially a filamentary material capable of carbonization without destruction of its filamentary nature. Organic materials which may be used for this purpose include natural and synthetic cellulose, which term include esters and ethers of cellulose, and polymers or copolymer of acrylonitrile. The steps in the production of the filament include in general a controlled pyrolysis step to form the carbonaceous material and a subsequent step of heating to well above the pyrolysis temperature.

In one process of this type a cellulosic material is subjected to a carefully controlled heating schedule in a protected atmosphere. This schedule has the aim of raising the temperature slowly from room temperature to about 400 C. to decompose the chemical structure of the cellulosic material without destroying its filamentary nature, then somewhat quicker increase of temperature up to about 900 C. to remove further more stable constituents of the pyrolyzed material and produce a filament component substantially of carbon, and finally a rapid heating up to about 3,000 C. to complete the pyrolysis and generate a stable carbon filament which may contain graphitic constituents.

In another process of this type a polymer or copolymer of acrylonitrile preferably containing at least 85 mole percent of acrylonitrile is wound on a frame and heated in the presence of oxygen at 220 C. to break down the chemical structure of the polymer and form a blackened filament which can then be pyrolyzed by subsequent heating in a protective atmosphere, for example argon, up to about 3,000 to form a stable carbon filament which may contain graphitic constituents. This subsequent heating may take place in two stages the first involving heating to about 900-l ,500 C., to produce a product of high strength, the second optional heating stage being heating to about 3,000 C. to produce a product of high modulus but somewhat lower strength than that product which results from the first heating stage.

The observation which we have now made is that the properties of the carbonaceous filament are subject to variation having regard to the conditions used for heating the material above about 900 C. and that the properties are improved if the heating is carried out under such conditions that trace active ingredients reactive with the carbon filaments, such as oxygen, which cannot be completely avoided, even with the use of a protective atmosphere are substantially eliminated from the environment surrounding the filament being heated.

According to the invention there is provided a process for improving the properties of carbon filaments formed by carbonization and pyrolysis of filamentary material capable of carbonization to temperature in excess of 900 C. with maintenance of filamentary structure which comprises the step of conducting the heating at above 900 C. in the presence of a carbon getter-ing material in a protective atmosphere.

The elimination of the active materials from the heating environment may be achieved, in one embodiment of the invention by packing the carbonaceous filaments, after pyrolysis of the carbonaceous forming material, with a carbonaceous gettering material, the packed filaments then being heated in a protective atmosphere above 900 C.

The carbonaceous gettering material may be added or "inherently provided in the environment. Thus, it may be added carbon black or it may be the filamentary carbonaceous material itself if the filaments are packed tightly together and the available space in the heating chamber is filled.

According to the invention not only can material of high strength be obtained without the necessity of elaborate gas purging and purification systems, but by using high packing densities as described above not only are the tensile strength properties of the product improved but the throughout obtainable from the heating chamber is also increased.

The obtaining of high packing density may however present problems since the filamentary material used generally has a very small diameter in the range 5-l0 micron and must be kept free from bends, kinks and loops if the filaments are to be successfully andusefully incorporated into matrices to form structural units. We have found this difficulty may be avoided if the filaments are wrapped in a disposable carbonaceous wrapper such as a strong wrapping paper, e.g. brown paper (kraft paper) or the like substantially free from fillers or waxy binders to form packages which can be fitted closely into the heating chamber. The filaments should be wrapped so that they do not cross over each other to produce small kinks but they need not be kept completely straight. If the heating chamber is a cylindrical chamber the filaments can be cut to a length slightly less than the length of the cylindrical furnace and tightly rolled in the disposable carbonaceous wrapper to form cylinders which can be easily packed into the furnace.

By following this technique the difficulty of having to pack the filaments tightly, with risk of damage is avoided, the carbonaceous gettering material being provided in situ.

In order that the invention may be more fully understood, the following example is given by way of illustration only:

EXAMPLE A cylinder 3 ft. long and 4 inches diameter was formed by wrapping a large number of carbonaceous filaments of length 3 ft. and diameter. 8 micron in brown paper. The carbonaceous filaments had been obtained by winding long monofilaments of polyacrylonitrile fiber of composition 93 weight percent acrylonitrile, 6 wt. percent methyl methacrylate and 1 wt. percent itaconic acid, onto a frame under an applied tension of 0.0002 kilograms per filament and pyrolyzing the filaments at 220 C. in the presence of oxygen until the filaments blackened followed by heating the filaments to 900 C. in the absence of oxygen, that is, in a protective atmosphere to 900C. to drive off more stable volatile constituents.

The wrapped cylinders so made were packed into the furnace and 99.9 percent pure nitrogen was passed through the furnace to purge the atmosphere and the temperature was thenraised to 2,600 C. During this heating argon was passed continuously through the furnace.

The fibers so produced, when tested had an ultimate tensile strength of 270,000 p.s.i. and a modulus of elasticity of 57x10 6 p.s.i.

in contrast with this, when the fibers were packed loosely into the furnace and filled only part of the furnace, the fibers so produced had an ultimate tensile strength of only l70,000 p.s.i. even though the modulus of elasticity was 63 l0 p.s.i., that is it was substantially unchanged.

We claim:

I. In a method for the production of carbon filaments by the pyrolysis of a filamentary material capable of carbonization without destruction of its filamentary nature when heated to a temperature of above 900 C., the improvement which comprises conducting the heating of the filaments above 900 C. by wrapping a plurality of them in a cellulosic paper to form a tightly rolled cylinder and heating said cylinder to a temperature in excess of 900 C. in the absence of oxygen.

2. A method as claimed in claim 1 in which the cellulosic paper is kraft paper.

3. A method as claimed in claim 1 in which the organic material is a polyacrylonitrile or a copolymer of acrylonitrile containing at least percent acrylonitrile.

4. A method as claimed in claim 2 in which the organic material is a polyacrylonitrile or a copolymer of acrylonitrile containing at least 85 percent acrylonitrile. 

2. A method as claimed in claim 1 in which the cellulosic paper is kraft paper.
 3. A method as claimed in claim 1 in which the organic material is a polyacrylonitrile or a copolymer of acrylonitrile containing at least 85 percent acrylonitrile.
 4. A method as claimed in claim 2 in which the organic material is a polyacrylonitrile or a copolymer of acrylonitrile containing at least 85 percent acrylonitrile. 